Nitric oxide (NO) is detectable in the exhaled air of human subjects, and its concentration is increased in patients with asthma. We have investigated the origin of the increase in exhaled NO in asthmatic patients by using different expiratory maneuvers and by direct sampling from the upper and lower respiratory tracts. Exhaled NO was measured by a chemiluminescence analyzer. Concentrations of NO measured during expiration against the resistance of the analyzer with exhaled flow of 1 L/min, were 78 +/- 3 ppb in normal subjects (n = 46) and significantly elevated in patients with asthma (301 +/- 26 ppb, n = 30, p < 0.001). Values of exhaled NO were lower when measured during unobstructed expiration with a flow of 5 L/min with sampling from a side-arm (7 +/- 1 ppb), but again were elevated in patients with asthma (46 +/- 6 ppb, p < 0.001). Breath-holding for 20 s resulted in an initial peak of NO, but end-expiration values similar to the unobstructed expiration. The concentration of NO in the nose was considerably greater than in expired air (996 +/- 39 ppb) and was elevated in patients with asthma (1,390 +/- 71 ppb, p < 0.002). Direct sampling from trachea and right middle lobe bronchus via a fiberoptic bronchoscope gave similar values in five normal and 15 asthmatic subjects to the values recorded during unobstructed expiration, and there was a good correlation between values in expired air and direct sampling (trachea r = 0.91, right middle lobe r = 0.87, p < 0.001). We conclude that exhaled NO measured in an unobstructed breath reflects concentrations in the lower respiratory tract, but that breath-holding or expiration against resistance is contaminated by residual NO derived from the upper respiratory tract. We also provide evidence that the elevated levels of exhaled NO in asthmatic patients are derived predominantly from the lower respiratory tract.